The purpose of this study is: 1) to determine the maximally tolerated dose (MTD) of lometrexol when given every three weeks to patients receiving 5 mg of folic acid IV, 1 hr preceding lometrexol and to describe the toxicities at each dose schedule studied and 2) to evaluate the pharmacokinetics of lometrexol in patients receiving exogenous folates, and compare them with pharmacokinetics in preceding trials.